|Working Group I: The Scientific Basis|
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7.6.5 El Niño-Southern Oscillation (ENSO)
The strongest natural fluctuation of climate on interannual
time-scales is the El Niño-Southern Oscillation (ENSO) phenomenon, and
ENSO-like fluctuations also dominate decadal time-scales (sometimes referred
to as the Pacific decadal oscillation). ENSO originates in the tropical Pacific
but affects climate conditions globally. The importance of changes in ENSO as
the climate changes and its potential role in possible abrupt shifts have only
recently been appreciated. Observations and modelling of ENSO are addressed
in Chapters 2, 8 and 9;
here the underlying processes are discussed. Observational and modelling results
suggest that more frequent or stronger ENSO events are possible in the future.
Because social and ecological systems are particularly vulnerable to rapid changes
in climate, for the next decades, these may prove of greater consequence than
a gradual rise in mean temperature.
ENSO is generated by ocean-atmosphere interactions internal to the tropical Pacific and overlying atmosphere. Positive temperature anomalies in the eastern equatorial Pacific (characteristic of an El Niño event) reduce the normally large sea surface temperature difference across the tropical Pacific. As a consequence, the trade winds weaken, the Southern Oscillation index (defined as the sea level pressure difference between Tahiti and Darwin) becomes anomalously negative, and sea level falls in the west and rises in the east by as much as 25 cm as warm waters extend eastward along the equator. At the same time, these weakened trades reduce the upwelling of cold water in the eastern equatorial Pacific, thereby strengthening the initial positive temperature anomaly. The weakened trades also cause negative off-equatorial thermocline depth anomalies in the central and western Pacific. These anomalies propagate westward to Indonesia, where they are reflected and propagate eastward along the equator. Thus some time after their generation, these negative anomalies cause the temperature anomaly in the east to decrease and change sign. The combination of the tropical air-sea instability and the delayed negative feedback due to sub-surface ocean dynamics can give rise to oscillations (for a summary of theories see Neelin et al., 1998). Two of these feedbacks are schematically illustrated in Figure 7.8. Beyond influencing tropical climate, ENSO seems to have a global influence: during and following El Niño, the global mean surface temperature increases as the ocean transfers heat to the atmosphere (Sun and Trenberth, 1998).
The shifts in the location of the organised rainfall in the tropics and the associated latent heat release alters the heating patterns of the atmosphere which forces large-scale waves in the atmosphere. These establish teleconnections, especially the PNA and the southern equivalent, the Pacific South American (PSA) pattern, that extend into mid-latitudes altering the winds and changing the jet stream and storm tracks (Trenberth et al., 1998), with ramifications for weather patterns and societal impacts around the world.
Another related feedback occurs in the sub-tropics. The normally cold waters off the western coasts of continents (such as California and Peru) encourage the development of extensive low stratocumulus cloud decks which block the Sun, and this helps keep the ocean cold. A warming of the waters, such as during El Niño, eliminates the cloud deck and leads to further sea surface warming through solar radiation. Kitoh et al. (1999) found that this mechanism could lead to interannual variations in the Pacific Ocean without involving equatorial ocean dynamics. Currently, stratocumulus decks are not well simulated in coupled models, resulting in significant deviations of SST from the observed (see Chapter 8, Figure 8.1).
Indices of ENSO for the past 120 years (Figure 7.9), indicate that there is considerable variability in the ENSO cycle in the modern record. This variability has been variously attributed to: (i) stochastic forcing due to weather and other high-frequency “noise”, and the Madden-Julian intra-seasonal oscillation in particular; (ii) deterministic chaos arising from internal non-linearities of the tropical Pacific ENSO system; (iii) forcing within the climate system but external to the tropical Pacific, and (iv) changes in exogenous forcing (see Neelin et al., 1998 and references therein). Palaeo-proxies, archaeological evidence, and instrumental data (see Chapter 2) all indicate variations in ENSO behaviour over the past centuries, and throughout the Holocene. Much of this variability appears to be internal to the Earth’s climate system, but there is evidence that the rather weak forcing due to orbital variations may be responsible for a systematic change to weaker ENSO cycles in the mid-Holocene (Sandweiss et al., 1996; Clement et al., 1999; Rodbell et al., 1999). However, it appears that the character of ENSO can change on a much faster time-scale than that of small amplitude insolation change imposed by the Earth’s varying orbit. The inference to be drawn from observed ENSO variability is that small forcings are able to cause large alterations in the behaviour of this non-linear system.
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